The present invention relates to the fabrication of liquid crystal devices. It particularly relates to liquid crystal displays having a liquid crystal layer disposed on an alignment layer that affects substantially the alignment of molecules in the liquid crystal layer. However, the invention will also find application in conjunction with other liquid crystal applications.
Liquid crystals are used in numerous display devices, such as notebook computers, desktop monitors, cellular telephone displays, high definition television, and the like, and in other photonic devices such as optical multiplexing coupler, switches, data storage, and so forth. The liquid crystal display typically includes a thin liquid crystal layer sandwiched between a pair of substrates of glass or another substantially light transmissive material. At least one of the substrates must be transparent. The display usually also includes one or two optical polarizer layers that cooperate with the liquid crystal layer and with biasing electronics to locally optically modulate optical path length of the LC film that in turn determines the opacity or reflectance of the liquid crystal display and changes pixel intensity.
In an active matrix liquid crystal display, independently addressable thin film transistors are fabricated on the substrate to serve as the biasing electronics. In backlit displays, a backlight is disposed behind the liquid crystal display, and the biasing electronics locally modulate opacity of the liquid crystal display to darken or brighten pixels. In reflective displays, the reflectance of the display is modulated. Color filters matched with primary color sub-pixels are included in color displays. Moreover, some liquid crystal displays employ a flexible substrate material such as a polymer film or flexiglass to provide a flexible display.
Regardless of the specific configuration and the type of liquid crystal display, a common element is one or more alignment surfaces that bias molecules of the liquid crystal toward a selected spatial alignment or orientation. A well known approach to forming the alignment surface is the rubbing method, in which a polyimide or other polymeric film is deposited on the substrate and physically rubbed using a velvet cloth to produce a directional or anisotropic template for molecules of the liquid crystal. The rubbing method is convenient and widely used in the industry; however, the method has substantial disadvantages, including a high potential for contamination, mechanical defects and damage, static charge generation which can damage the transistors in active matrix displays, and difficulty of obtaining uniformity in rubbing strength over large areas.
As the liquid crystal display industry moves toward larger area and higher resolution displays, there has been an increasing desire to develop an improved method for forming the alignment surface which does not involve physically contacting the substrate. For example, U.S. Pat. No. 5,770,826 issued to Chaudhari et al. discloses a non-contact method that uses a low energy ion beam to define the alignment surface. Other methods include deposition of a Langmuir-Blodgett film, oblique angle deposition of silicon oxide or other inorganic materials, exposure of a polymer film to polarized ultraviolet radiation, and plasma irradiation.
While these methods improve upon the rubbing method by eliminating physical contact, they have a number of disadvantages. Direct formation of an alignment surface that is uniform over large areas by direct deposition of an anisotropic alignment layer is difficult, especially for substrate areas on the order of several square meters which are preferred for large-area displays and for high manufacturing throughput. The ion beam and plasma irradiation methods are both performed in a vacuum environment, which is difficult to achieve over a large-area substrate and reduces manufacturing throughput. Moreover, as these methods are performed prior to sealing of the liquid crystal film, they can introduce contamination that degrades the liquid crystal display.
The present invention contemplates an improved apparatus and method which overcomes the aforementioned limitations and others. In place of rubbing, deposition, UV or plasma exposure, this method uses exposure to electron beam, which can be performed at ambient conditions. The exposure modifies the surface properties that causes the liquid crystal molecules to anchor at specific orientations with respect to the substrate.